Since the early 1900's there have been numerous attempts to develop a practical multi-ratio drive system to mechanically transmit power between two rotating shafts where the drive ratio can be varied under a load. A simple multi-ratio drive has numerous applications in both the motor vehicle and the bicycle industries. Much attention has been focused upon variable diameter pulleys which engage conventional V-belts, as shown in U.S. Pat. No. 4,342,559 -- Williams. Some work has also been done in the chain area using variable diameter sprockets. An early example of a variable diameter sprocket mechanism is shown in U.S. Pat. No. 1,428,999 -- Tygard. The Tygard patent employs a sprocket having a plurality of concentric rows of cylindrical pins which translate axially between an operative and inoperative position to vary the effective sprocket diameter. The pins which are mechanically shifted fore and aft directly engage a chain which has been adapted to fit the pins. A more recent version of the Tygard mechanism is shown in U.S. Pat. No. 4,457,739 -- Iseman, which is incorporated by reference herein. Since the Iseman transmission is adapted to be used in a bicycle, the sprocket assembly is much thinner, there are more ratios, and the reverse gear of Tygard is eliminated.
There are a number of mechanical problems associated with variable ratio sprockets and chains of the type shown in Tygard and Iseman which have hindered commercialization of these mechanisms. The principal problems are related to: pin shifting, chain-pin interference, chain ratcheting and the chain tensioning mechanism. During each revolution of the sprocket, a chain link must engage and disengage a pin. The link must be designed so that interference is kept to a minimal level at all sprocket diameters and chain loads. The Tygard patent discloses a generally C-shaped link with a large amount of link chain interference which will increase with the load as the link deforms. The Iseman chain is designed to operate over a wide range of sprocket diameters, resulting in moderate interference and a tendency to ratchet, i.e., chain slippage, across the pins, at large sprocket diameters. In order to avoid ratcheting, it is necessary to maintain high chain slack side tension.